Fuel injectors are required to be able to undergo hundreds of millions of on/off cycles and still meet the original fluid flow rates and leak performance specifications. The failure to meet and maintain such original performance specifications will result in varying fuel metering to the engine. Some compensation can be made in the engine control system for the overall lean or rich composition of the fuel charge, but for a lean or rich cylinder, such compensation is not always practical. When this happens, the engine may well be unable to meet emission and performance expectations.
The cause of such lean or rich mixtures in a given cylinder can be cause by many factors, one of which is the accuracy of the guiding mechanism for the armature/needle in its reciprocal motion on and off the valve seat. Traditionally injectors have been guided with at least a two point guiding scheme with one guide at the upper end of the armature/needle close to the `power group` of the injector and the other at the lower end nearer the valve seat.
Still another cause of such performance may be traced to the sealing members in the injector which can cause misalignment of the armature/needle.
Some traditional methods of creating the guide mechanism include utilizing the bore of the valve body for both upper and lower guides. This requires the bore inner diameter to be machined to closely controlled tolerances and then the outer surfaces of the armature/needle are also machined to tight tolerances. Even with this, there may be a required sizing and matching manufacturing operation. Again typically when this is done, the sealing area in the seat of the valve body is also tightly machined to match the sealing area on the pintle valve member or needle valve member depending on the type of valving the injector uses.
Other methods to avoid any misalignment include utilizing a spherical shaped ball geometry at the needle valve member's end as the lower guide. In this case the armature outer diameter guides on a machined surface in the valve body which functions as the upper guide. This is shown in U.S. Pat. No. 5,217,204. This type of design has a guiding advantage due to the ability of a spherical geometry to pivot, but it does require extensive machining in the seat area. Additionally either the seat or the surface of the sphere requires machining to achieve the desired flow passage to the metering area of the valve.
In such an instance as above, the valve body is part of the magnetic return path and therefore, the surface that the armature guides on must be non-magnetic to minimize friction resulting from the magnet's attraction. To accomplish this, there is a separate piece that is attached to the valve body and then machined with the valve body to insure centering of the armature/needle. This requires very close tolerance machining over an extended distance.
As taught in U.S. Pat. No. 4,915,350, one method to solve this has been sizing and attaching a non-magnetic thin guide onto the top of the valve body. This will lower the cost due to the minimization of a machining that is required. Some drawbacks are if the actual attachment of the guide is with a staking operation, such operation creates small metallic particles which can potentially be a source of injector contamination and subsequent injector failure. If the thin guide is located in a recess in the valve body, an additional machining operation must be performed on the valve body to accept a portion of the thickness of the already thin guide. To allow for the stacking, the valve body must have additional space in the diameter to accommodate the material necessary for the geometry for the retention of the guide.
In other applications, although the guide is sized during the attaching operation, the centering of the upper guide to the lower guide is dependent upon the tolerances built into the valve body. The guide conforms to the position of the valve body inner diameter. When the sizing tool is removed, the lower end of the guide has a tendency to spring back some due to the properties of the metal. This leaves a potential sharp area to gouge into the armature.